Ultrastructural Changes in the Ovarian Follicular Wall During Oocyte Growth in the Nile Tilapia, Oreochromis niloticus Linn.

Abstract

Ultrastructural study of cytodifferentiation in the ovarian follicle of the Nile tilapia, Oreochromis niloticus, was carried out during the period of oocyte growth with special reference to the vitelline envelope, granulosa and thecal cell layers. In the perinucleolar stage, a simple layer of flattened granulosa cells was observed. The cells possess multivesicular bodies, RER and free ribosomes. In the cortical alveolar stage, granulosa cells also show a well developed dilated tubular RER and electron-dense materials. At this stage the vitelline envelope is observed as a single electron-dense mesh pattern layer, becoming thicker during the vitellogenic stage. From the early vitellogenic stage, the amount of electron-dense materials in granulosa cells increased. The granulosa cells proliferate and form a multicellular layer. The cells are organelle-rich, with elongate mitochondria, free ribosomes, dilated tubular RER and a Golgi system. The theca of the perinucleolar oocyte is a stratified squamous layer. At this stage, the thecal cells show ultrastructural steroidogenic features including mitochondria with tubular cristae, abundant globular SER and transport vesicles with an electron-dense content. In the vitellogenic stage, the thecal cells still show steroidogenic characteristics, but with more abundant mitochondria with tubular cristae as well as some pleomorphic mitochondria. Overall, these results describing the ultrastructure of the developing O. niloticus oocyte focusing on the morphology of the follicular cells suggest a primary role for thecal cells in production and secretion of steroid hormones at this stage of ovarian development in this species.